Fix bad selector; this was breaking preview tab.

themes/bootstrap3/js/preview.js

@@ -72,7 +72,7 @@ function processGBSBookInfo(booksInfo) {
         if (viewOptions.tab.indexOf(booksInfo[bibkey].preview) >= 0
         && (booksInfo[bibkey].embeddable)) {
           // make tab visible
-          $('ul.nav-tabs li.hidden a.preview').parent().removeClass('hidden');
+          $('ul.nav-tabs li.hidden[data-tab="preview"]').removeClass('hidden');
         }
       }
     }